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DCCCORNER Light passenger car interiors with DCC Walthers passenger cars come equipped with factory-installed electrical pickups, and the firm sells separate lighting kits for both DC and DCC. The kits are easy to install – just remove the roof and set the light bar in place. The DC kit (933-1087) uses a 5V reg- ulator to lower the voltage. This provides constant lighting if the track voltage is above 6V. The DCC version (933-1088) uses three bulbs in series so they equally divide the voltage. On a typical 14V DCC system, each bulb operates on a little less than 5V. I had several Walthers coaches to Mike Polsgrove explains how he added DCC-operated interior lighting to this Walthers HO scale heavyweight passenger car. Mike Polsgrove photos If you have a direct-current (DC) model railroad, you’ve probably struggled to provide constant lighting for your passenger cars. Since the track voltage controls train speed in DC, when your train is stopped at the station, the passengers are left in the dark. Digital Command Control provides a constant track voltage. This means that whether your train is making a station stop or rolling down the tracks, the interior lighting will remain at the same brightness. However, you must take into account the extra load of the lights when calculating the size of your DCC booster, as it supplies all the power for them. This month, I’ll show you how I added DCC-operated interior lighting to a Walthers heavyweight passenger car. The techniques can be easily adapted for use in cars by other manufacturers. Getting started. Most of today’s passenger cars are equipped with electrical contacts for interior lighting. If your model doesn’t have contacts, you can modify most plastic trucks by adding metal wheelsets and simple pickup wipers or shoes [See “Decoder-controlled caboose lighting” in the March 2006 Model Railroader for more on installing wipers. – Ed.] Track voltage provided by a booster varies depending on the system, but most DCC systems range from 12-16 volts (V). The most accurate way to determine the voltage is by measuring it. Though most digital voltmeters aren’t designed to measure a DCC signal, you can get a rough estimate of the voltage using the AC setting. Some DCC systems provide an internal DC measuring point. There are also external circuits and meters designed specifically for DCC that could give you exact figures, but that’s usually not necessary. Light the lights. Light bulbs will operate on either DC or the AC signal of a DCC system. If you use an 18V bulb connected directly across the rails, you’ll likely be happy with the results. An alternative is to use a low-voltage bulb with a dropping resistor. Calculating the value of the resistor is the same as it is for a locomotive headlight, which I covered in last month’s column. Light-emitting diodes (LEDs) may also be used to light passenger car interiors, but they won’t operate on AC. Though the LED will rectify the AC signal into a DC signal, DCC voltage often exceeds the diode’s Peak-Inverse-Voltage (PIV). To prevent the DCC voltage from exceeding the PIV, use a bridge rectifier and a current-limiting resistor. 66 Model Railroader • www.ModelRailroader.com equip with interior lighting, so I purchased the DCC lighting kits. I also wanted the ability to turn the lights on and off, so I added a DCC decoder to each car. I chose a Digitrax TF4 function-only decoder because it’s compact and has four function outputs. Train Control Systems (TCS) also makes lighting-only decoders, however, you can use just about any decoder that has function outputs. Decoder installation. The Walthers light bar has two rivets that make contact with copper strips at one end to provide track power to the top of the car, as shown in fig. 1. A small printed-circuit (PC) board connects the lights in series. To start, I removed all the bulb wires from the PC board. I then soldered one wire from each bulb to an unused section of the PC board. (See fig. 1.) The Digitrax decoder’s blue wire is the lighting common. I soldered the blue wire to the PC-board as well, connecting it to the three bulb wires already there. Next, I soldered the red and black decoder wires to the wires from the PC board’s contact rivets, also shown in fig. 1. In order to properly illuminate the bulbs with a decoder output on my DCC system, I needed to drop the voltage about 7V. The function-output voltage can be measured directly with a DC voltmeter when the decoder is connected to the track and the function is turned on. The blue wire is positive and the function outputs are negative. I also measured the current each bulb drew by placing a multimeter in series with the bulbs and powering it from a